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Effect of Thermal Bridges in Insulated Walls on Air-Conditioning Loads Using Whole Building Energy Analysis

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Effect of Thermal Bridges in Insulated Walls on Air-Conditioning Loads Using Whole Building Energy Analysis sustainability Article Effect of Thermal Bridges in Insulated Walls on Air-Conditioning Loads Using Whole Building Energy Analysis Mohamed F. Zedan *, Sami Al-Sanea, Abdulaziz Al-Mujahid and Zeyad Al-Suhaibani Mechanical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia; [email protected] (S.A.-S.); [email protected] (A.A.-M.); [email protected] (A.A.-S.) * Correspondence: [email protected]; Tel.: +966-50-895-4548 Academic Editor: Andrew Kusiak Received: 13 April 2016; Accepted: 9 June 2016; Published: 16 June 2016 Abstract: Thermal bridges in building walls are usually caused by mortar joints between insulated building blocks and by the presence of concrete columns and beams within the building envelope. These bridges create an easy path for heat transmission and therefore increase air-conditioning loads. In this study, the effects of mortar joints only on cooling and heating loads in a typical two-story villa in Riyadh are investigated using whole building energy analysis. All loads found in the villa, which broadly include ventilation, transmission, solar and internal loads, are considered with schedules based on local lifestyles. The thermal bridging effect of mortar joints is simulated by reducing wall thermal resistance by a percentage that depends on the bridges to wall area ratio (TB area ratio or Amj/Atot) and the nominal thermal insulation thickness (Lins). These percentage reductions are obtained from a correlation developed by using a rigorous 2D dynamic model of heat transmission through walls with mortar joints. The reduction in thermal resistance is achieved through minor reductions in insulation thickness, thereby keeping the thermal mass of the wall essentially unchanged. Results indicate that yearly and monthly cooling loads increase almost linearly with the thermal bridge to wall area ratio. The increase in the villa’s yearly loads varies from about 3% for Amj/Atot = 0.02 to about 11% for Amj/Atot = 0.08. The monthly increase is not uniform over the year and reaches a maximum in August, where it ranges from 5% for Amj/Atot = 0.02 to 15% for Amj/Atot = 0.08. In winter, results show that yearly heating loads are generally very small compared to cooling loads and that heating is only needed in December, January and February, starting from late night to late morning. Monthly heating loads increase with the thermal bridge area ratio; however, the variation is not as linear as observed in cooling loads. The present results highlight the importance of reducing or eliminating thermal bridging effects resulting from mortar joints in walls by maintaining the continuity of the insulation layer in order to reduce energy consumption in air-conditioned buildings. Keywords: thermal bridges; mortar joints; transmission load; R-value; wall thermal performance; building envelope 1. Introduction Thermal bridges in the outer walls of buildings are typically caused by mortar joints between insulated building blocks and by concrete structural elements, such as columns, beams and slabs. These bridges create an easy path for heat transfer across the building envelope and usually result in increased heating and cooling loads in buildings. Accurate thermal bridging assessment is becoming extremely important not only in predicting peak thermal loads and yearly heating/cooling loads, but also in estimating the potential for condensation and subsequent mold growth in the heating season in Sustainability 2016, 8, 560; doi:10.3390/su8060560 www.mdpi.com/journal/sustainability Sustainability 2016, 8, 560 2 of 20 cold climates. The main difficulty in modeling thermal bridges stems from the fact that they create significantSustainability two-dimensional 2016, 8, 560 (2D) effects on heat transmission across building walls in the2 of case 21 of mortar joints and three-dimensional (3D) effects in the case of concrete structural elements. At the samesignificant time, most two-dimensional building energy (2D) simulation effects on computer heat transmission packages across utilize building one-dimensional walls in the case models of in ordermortar to simplify joints lengthyand three-dimensional calculations. (3D) effects in the case of concrete structural elements. At the same time, most building energy simulation computer packages utilize one-dimensional models in The literature survey reported in the next section shows that most studies on thermal bridges order to simplify lengthy calculations. consider mainlyThe literature those caused survey byreported structural in the elements next sect andion shows that little that attention most studies was on given thermal to mortar bridges joints. To remedyconsider this mainly deficiency, those thecaused present by structural investigation elemen dealsts and with that the little effects attention of mortar was given joints to in mortar the walls of a typicaljoints. To villa. remedy When this using deficiency, insulated the present building investigation blocks, mortar deals with joints thecause effectsdiscontinuities of mortar joints in the insulationthe walls layer(s) of a typical along villa. the wall When height, using asinsulated shown building in Figure blocks,1, and mortar therefore joints provide cause discontinuities little resistance to heatin transmissionthe insulation comparedlayer(s) along to thethe insulationwall height, layer as shown itself. in This Figure affects 1, and adversely therefore theprovide effectiveness little of thermalresistance insulation, to heat resultingtransmission in considerablecompared to the reduction insulation in layer theR-value itself. This of theaffects wall adversely compared the to a wall witheffectiveness a continuous of thermal insulation insulation, layer. resulting Using in a 2Dconsiderable dynamic reduction model for in heat the R-value transmission of the throughwall compared to a wall with a continuous insulation layer. Using a 2D dynamic model for heat a typical wall section with mortar joints, Al-Sanea and Zedan [1] showed that the reduction in the transmission through a typical wall section with mortar joints, Al-Sanea and Zedan [1] showed that R-value depends on both the ratio of the thermal bridge area to wall area (TB ratio or A /A ) and the reduction in the R-value depends on both the ratio of the thermal bridge area to wall mjarea (TBtot the insulation thickness (L ). In the present paper, we deal with a whole building (villa) rather than a ratio or Amj/Atot) and insthe insulation thickness (Lins). In the present paper, we deal with a whole wall sectionbuilding [1 (villa)] to study rather the than effects a wall of section thermal [1] bridges to study on the the effects hourly, of thermal monthly bridges and yearly on the coolinghourly, and heatingmonthly loads, and under yearly Riyadh cooling ambient and heating conditions. loads, Allunde loadsr Riyadh found ambient in a typical conditions. villa All are loads considered found in using wholea buildingtypical villa energy are considered analysis. Theseusing whole loads broadlybuilding includeenergy analysis. ventilation These loads, loads solar broadly loads, include internal loadsventilation (lights, people, loads, equipment,solar loads, internaletc., with loads typical (lights, schedules people, equipment, based on localetc., with lifestyles) typical andschedules of course heat transmissionbased on local through lifestyles) the and villa’s of course envelope, heat transm withission the latter through being the thevilla’s dominant envelope, load with under the latter Riyadh ambientbeing conditions. the dominant load under Riyadh ambient conditions. Hmj Mortar joint Masonry Hb Insulation Masonry with air space Outside Inside FigureFigure 1. Diagrammatic 1. Diagrammatic sketch sketch showing showingthermal thermal bridges ca causedused by by mortar mortar joints joints in building in building walls. walls. The thermal bridging effect resulting from mortar joints is simulated, in the present work, by Thereducing thermal the wall bridging thermal effect resistance resulting (R-value) from by mortar a percentage joints that is simulated, corresponds in to the the presentbridge to work, wall by reducingarea theratio wall and thermal the nominal resistance thickness (R-value) of the insulati by a percentageon layer. The that percentage corresponds reduction to thein the bridge R-value to wall area ratiois obtained and the from nominal a graphical thickness correlation of the developed insulation from layer. detailed The percentage and rigoro reductionus 2D dynamic in the analysis R-value is obtainedof heat from transmission a graphical through correlation the same developed wall section from that detailed includes and mortar rigorous joints, 2D in dynamica way similar analysis to of heat transmissionthe study presented through by thethe sameauthors wall in sectionAl-Sanea that and includes Zedan [1]. mortar The reduction joints, in in a waywall R-value similar tois the studyachieved presented by by reducing the authors the insulation in Al-Sanea thickness and Zedan and thereby [1]. The keeping reduction the thermal in wall mass R-value of the is achievedwall by reducingessentially the unchanged. insulation thicknessThe whole and building thereby lo keepingad simulation the thermal package mass used of thein wallthe present essentially Sustainability 2016, 8, 560 3 of 20 unchanged. The whole building load simulation package used in the present investigation is the Hourly Analysis Program provided to the authors by the Carrier Corporation. This package, which is known as
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